What are Carbohydrates?

Carbohydrates are biological molecules containing carbon, hydrogen, and oxygen atoms, typically in a 1:2:1 ratio. For example, glucose has six carbons, twelve hydrogens, and six oxygens (C6H12O6). This elemental composition defines all carbohydrates, from simple sugars to complex polymers, making them distinct from other macromolecules.

Monosaccharides are classified by carbon count: pentoses have five carbons, hexoses have six. They are also classified by carbonyl group placement. An aldose has a carbonyl group at the end of the molecule, while a ketose has it in the middle. Glucose is an aldohexose, and fructose is a ketohexose.

D-glucose and L-glucose are enantiomers, mirror images of each other, differing in the orientation of the hydroxyl group on carbon five. If the hydroxyl group points right, it is D-glucose; if it points left, it is L-glucose. This spatial arrangement affects how the molecules interact biologically.

Two monosaccharides combine through dehydration synthesis, a chemical reaction that removes water and forms a covalent bond. For example, glucose and fructose join to create sucrose, the common table sugar. This process links the sugar molecules into a larger disaccharide structure.

Polysaccharides are complex carbohydrates formed when many monosaccharides link together through repeated dehydration synthesis reactions. Cellulose and amylose are common polysaccharides built from glucose monomers. Cellulose is insoluble and serves as the structural component of plant cell walls and fibers.

Carbohydrates serve two major functions in cells: they act as energy reserves, providing fuel for cellular processes, and they function as structural components. Simple sugars like glucose are readily used for energy, while polysaccharides like cellulose provide structural support in plant cell walls.

The Haworth projection represents a monosaccharide as a ring structure, revealing how atoms arrange in three-dimensional space. It shows whether the hydroxyl group on the carbonyl carbon points down (alpha form) or up (beta form). This representation clarifies the stereochemistry that distinguishes different sugar isomers.